The analog front end of an RFID transponder includes a resonant LC antenna circuit. In high frequency RFID transponders the resonant frequency is typically 13 MHz. The antenna captures the modulated RF field from an interrogator (ASK downlink modulation) giving rise to a fairly high antenna voltage with voltage swings in the range of 8V. An antenna voltage in the range of 8V allows a straight forward envelope detection for demodulation with a conventional CMOS circuit used in the RFID transponder.
However, advanced CMOS technologies allow only small drain-source and gate voltages. A voltage swing in a range of 8V is too high for integrated CMOS circuits implemented in an advanced CMOS technology, especially when a low cost process is used which does not support high voltage transistors or high precision capacitors for use in a resonant circuit. The antenna voltage needs to be limited by some voltage limiting circuit to comply with the specifications of the CMOS circuit. When a voltage limitation to a maximum of 3.6V is needed, the minimum needed supply voltage limits the antenna voltage down to 1.6V plus a (rectifier) diode voltage of 0.8V i.e. 2.4V. Taking into account inevitable CMOS process variations, the antenna voltage must be limited so as not to exceed a voltage range of 2.4V to 3.6V. Accordingly, the voltage swing from an ASK downlink modulation will be severely clamped, and a direct envelope demodulation will not work any more.
However, the ASK information is present in the limiter current. A possible approach to demodulation is to detect the gate voltage of the limiter transistors in the voltage limiting circuit since it also contains the modulation information. But the gate voltage of the limiter transistors is a quadratic function of the RF field and has to be linearized to ensure a defined sensitivity.